const months = ['一月','三月','五月','九月'];

months.splice(1, 0, '二月');
console.log(months);

months.splice(4, 1, '六月');
console.log(months);

let index = ( Math.random() * months.length ) | 0 ;
console.log( index );
months.splice( index ,1);

console.log(months);

